Method for coating polytetrafluoroethylene on material



United States Patent Ofi 3,514,314 Patented May 26, 1970 ice 3,514,314 METHOD FOR COATING POLYTETRAFLUORCh ETHYLENE N MATERIAL John B. Nemeth, Cheshire, C0nn., assignor to RDM Inc., Meriden, C0nn., a corporation of Connecticut No Drawing. Filed Apr. 10, 1967, Ser. No. 629,366 lint. Cl. 844d ]/22, 1/46, J/36 US. Cl. 11761 3 Claims ABSTRACT OF THE DISCLOSURE An elastomeric or metallic article is placed in an evacuated chamber and a Teflon dispersion is introduced into the chamber so that the dispersion enters the pores and/ or follicles in the surface of the article. With the article submerged in this dispersion, the chamber is pressurized, after which the article is removed and immediately Washed to remove the excess dispersion. The article is then dipped in a dilute aqueous solution of Teflon particles by weight) after which it is dried to evaporate the liquid, and then heated to sinter the Teflon into a wear resistant coating on the article surface.

BACKGROUND OF INVENTION The use of Teflon, or polytetrafluoroethylene, in finely divided form suspended in a colloidal solution with a vaporizable carrier liquid for coating porous metal surfaces is disclosed in Pat. 2,731,360 issued to Love in 1956. As disclosed therein, air is removed from the pores in the metal surface by evacuating a chamber to approximately one inch of mercury absolute pressure, the colloidal solution of paste-like consistency is allowed to cover the surface, and atmospheric pressure is used to drive the paste into the pores. The liquid carrier is then evaporated, as by the application of heat, after which the Teflon must be burnished or otherwise worked to provide a smooth outer surface for the coating.

In Pat. 3,155,441 issued to Bemmann in 1964, a variation of the above-described process of Love is disclosed in a bearing construction wherein the final finishing of the Teflon surface is accomplished by sizing the bearing while holding its inside diameter constant on a mandrel or the like.

SUMMARY OF INVENTION This invention relates to an improved method of depositing a Teflon coating on the surface of an elastomeric or metal article, wherein the Teflon can be sintered to yield a smooth surface without any necessity for burnishing or otherwise working the exposed Teflon surface.

A general object of the present invention is to provide a method of depositing Teflon on a porous metal surface, such as sintered bronze, with the result that a smooth bearing surface is formed directly from the Teflon dispersion deposited on the bearing surface, no subsequent burnishing or coining being required to smooth the exposed Teflon surface.

Another general object of the present invention is to provide a method of depositing Teflon on an elastomeric surface, such as rubber or silicone, with the resulting Teflon surface being very resistant to crazing or cracking in spite of flexing movement of the elastomeric material within or even well beyond the elastic limit of the Teflon material itself.

DETAILED DESCRIPTION According to one embodiment of the present invention, a sintered bronze bushing having an internal diameter several thousandths over the desired, or normal, hearing size is placed in an air-tight chamber from which the air is extracted by use of a vacuum pump to approximately one inch of mercury or less. The chamber is connected, through a line or conduit, with a reservoir having an aqueous solution in which finely divided Teflon particles are suspended. The line is provided with a petcock which is closed until the vacuum pump has removed most of the air. At this time, the petcock is opened and the slurry, or Teflon dispersion, is allowed to enter the chamber, atmospheric pressure being exerted on the reservoir at all times. Preferably, and in accordance with the teachings of the previously mentioned patents, this slurry or latex as it is sometimes called, comprises 40% to 70% by weight of Teflon particles, and the remaining portion comprises water, or any other readily volatizable liquid. Enough slurry is allowed to flow into the chamber so that the bushing is completely covered, at which time the above-mentioned petcock is closed and atmospheric pressure is restored in the chamber, as for example by shutting ofl the vacuum pump and opening a valve in the top of the chamber. The Teflon dispersion is thereby forced into the pores and cavities of the sintered bronze hearing by atmospheric pressure, and it will be apparent that super-atmospheric pressure might be applied to the slurry in the chamber if required.

The bronze bushing is then removed from the chamber and the excess slurry washed therefrom. The washing is preferably accomplished by immersing the bearing in cold water momentarily. After washing, the bushing is immediately dipped into a dilute solution of Teflon particles suspended in a vaporizable liquid carrier such as water. In accordance with the resent invention, the washing and dipping are carried out before any appreciable drying of the previously deposited dispersion has taken place, and it is an important feature of the present invention that the clipping solution through which the still wet bushing is passed comprises an aqueous solution having approximately 15% by Weight of Teflon particles suspended therein.

After being dipped, the bushing is dried in an oven at a temperature above F. but preferably below 300 F. For average size bushings having a wall thickness of less than one half inch, it has been found satisfactory to hold the parts to bedried at F. for approximately one half hour, at F. for another half hour, at 200 F. for a half hour, and at 225 F. for another half hour. It will of course be apparent that more massive parts might have to be left at somewhat higher temperatures for a correspondingly longer period of time.

Finally, the dried parts are baked in an oven at a temperature in the range between 600-800 F. to sinter the Teflon particles. Again, for average size bushings, it has been found that a temperature between 700-800 F. for approximately 15 minutes gives very smooth Teflon surfaces when carried out as the final step in the process outlined above. Thus, there is no requirement for working the resulting Teflon surface when the process disclosed herein is adhered to.

By way of example, a sintered bronze bearing having an inner diameter of 0.439 inch, a wall thickness of inch, and an axial length of one inch was coated with Teflon to a thickness of between .0002 to .0004 inch using the above-described process. A Teflon slurry having Teflon particles, 60% by weight, suspended in water was used and the bearing was immersed in cold tap water less than one minute after its removal from the chamber. Immediately after washing or rinsing, the bearing was dipped in a dilute aqueous solution, 15 by weight, of Teflon particles. The coated bearing was then dried according to the above schedule, after which it Was baked at 750 F. for 15 minutes.

According to an alternative embodiment of the present invention, the general method set forth above can be applied to coating Teflon on elastomeric materials such as fluorocarbon rubber or on silicone rubber. More particularly, the method to be described hereinbelow comprises a very satisfactory process for forming a continuous coating of Teflon on the surface of an O ring.

The rubber O ring is placed in an air-tight chamber from which the air is extracted by means of a conventional vacuum pump so that the pressure inthe chamber is reduced to approximately one inch of mercury or less. As in the previous embodiment, the chamber is connected, through a line or conduit, with a reservoir having an aqueous solution or slurry in which finely divided Teflon particles are suspended. A petcock or valve is provided in the line, which petcock is closed until the vacuum pump has removed substantially all of the air in the chamber. At this time, the petcock is opened and the slurry, or Teflon dispersion, is allowed to enter the chamher as a result of atmospheric pressure being exerted on the reservoir. Preferably, and in accordance with conventional practice, this slurry or latex as it is sometimes called, comprises 40% to 70% by weight of Teflon particles, and the remaining portion comprises water or any other readily volatilizable liquid. The slurry is allowed to flow into the chamber so that the O ring is completely covered, at which time the above-mentioned petcock is closed and atmospheric pressure is restored in the chamber, as for example by shutting off the vacuum pump and opening a valve in the top of the chamber. It has been found that the Teflon dispersion is thereby forced onto the surface of the rubber, and that atmospheric pressure enhances the mechanical adhesion of the Teflon to the surface of the O ring.

The O ring is then removed from the chamber and the excess slurry washed therefrom, after which washing the O ring is immediately dipped into a dilute solution of Teflon particles suspended in a vaporizable liquid carrier such as Water. In accordance with the present invention, the washing and dipping are carried out before any appreciable drying of the previously deposited dispersion has taken place, and it is an important feature of the present invention that the dipping solution through which the still wet O ring is passed comprises an aqueous solution having approximately 15% by weight of Teflon particles suspended therein.

After being so dipped, the O ring is dried in an oven at a temperature of approximately 225 F. While this temperature has been found suitable for coating rings of less than one half inch in wall thickness, it will be apparent that somewhat higher temperatures might be resorted to for carrying out the present process with somewhat larger articles. Preferably, the temperature used should lie in the range between 200 and 250 F. with the time period for accomplishing said heating being limited to approximately ten minutes in order to minimize the likelihood of damage to the underlying substrate rubber material.

Finally, the dried parts are baked in an oven at a temperature of at least 650 F. to sinter the Teflon particles. In order to minimize degrading of the rubber, this final heating is accomplished for no longer than a feW minutes. With average size 0 rings, a time of five minutes has been found to be satisfactory. The rather short time period assures that the temperature of the Teflon coating is raised to 650 F. without necessarily raising the temperature of the underlying rubber material high enough to cause degrading or deterioration due to excessive heat. It has been found that the above-described process for coating elastomeric material with Teflon yields a very smooth surface, with no subsequent working of the Teflon surface being required.

By way of illustrating the usefulness of the abovedescribed alternative method of coating an elastomeric material with Teflon, the following example is offered. An 0 ring seal of Viton (a fluorocarbon rubber) having an inner diameter of 1.984 inches and a bead diameter of 0.139 inch was successfully coated with Teflon to a thickness of .0005 inch. A Teflon slurry having 60% by weight of Teflon particles suspended in water was used and the O ring was immersed in cold tap water less than one minute after its removal from the chamber. Immediately after such washing, or rinsing, the O ring was dipped in a dilute aqueous solution, 15 by weight, of Teflon particles. The coated O ring was then dried in an oven at 225 i F. for 10 minutes, after which it was baked for 5 minutes as the oven temperature was raised to 650 F.

What is claimed is:

1-. A method of coating polytetrafluoroethylene onto the follicular surface of an article comprising the successive steps of:

.(at) removing air from the surface to be coated by I establishing sub-atmospheric pressure at said surface,

(b) introducing a liquid dispersion of polytetrafluoroethylene particles to said surface while maintaining the sub-atmospheric pressure,

(c) raising the pressure adjacent the surface to at least atmospheric pressure to drive the dispersion of polytetrafluoroethylene particles into said receptive surface, said liquid dispersion of polytetrafluoroethylene particles comprising an aqueous solution to paste-like consistency having 40%70% by weight of polytetrafluoroethylene,

(d) washing the excess dispersion off said surface,

(e) passing the surface through a dilute solution of polytetrafluoroethylene particles suspended in a vaporizable liquid carrier, said dilute solution of polytetrafluoroethylene particles comprising an aqueous solution having approximately 15% by weight of polytetrafluoroethylene particles suspended therein,

(f) drying the liquid from said surface by evaporating the carrier liquid at a temperature in the range of IOU-300 F.,

(g) and heating said coated surface to a temperature in the range of 600800 F. for a suflicient time to sinter the polytetrafluoroethylene particles in said coating.

2. The method according to claim 1 wherein the article comprises an elastomeric material, said heating step being accomplished at a surface temperature between 600 and 700 F. for long enough to sinter said polytetrafiuoro ethylene coating at the follicular surface of the material but no long enough to cause deterioration of the underlying elastomeric material.

3. The method according to claim 1 wherein the article is a porous metallic article.

References Cited UNITED STATES PATENTS 2,364,713 12/1944 HenSel 117119 X 2,731,360 1/1956 Love 1l7--61 X 2,764,505 9/1956 Kilbourne et a1. 117--161 X 2,838,829 6/1958 Goss et a1 117-119 X 2,944,917 7/1960 Cahne. 3,019,206 1/1962 Robb 1l7139 X 3,155,441 11/1964 Bemmann 308-241 3,294,568 12/1966 GOssInann 117-161 X 3,369,924 2/1968 Duggins et a1. 117--65.2

FOREIGN PATENTS 657,085 9/ 1951 Great Britain. 912,793 12/ 1962 Great Britain.

WILLIAM D. MARTIN, Primary Examiner HARRY J. GWINNELL, Assistant Examiner U.S. Cl. X.R. 

